Adhesives have been used for a variety of marking, holding, protecting, sealing and masking purposes. Adhesive tapes generally comprise a backing, or substrate, and an adhesive. One type of adhesive which is particularly preferred for many applications is represented by pressure sensitive adhesives. Pressure sensitive adhesives (PSAs) are well known to one of ordinary skill in the art to possess certain properties including the following: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength.
Materials that have been found to function well as pressure sensitive adhesives are polymers designed and formulated to exhibit the requisite viscoelastic properties resulting in a desired balance of tack, peel adhesion, and shear strength. The most commonly used polymers for preparation of pressure sensitive adhesives are various (meth)acrylate based copolymers, natural rubber, synthetic rubbers, and silicones.
Pressure sensitive adhesive foams, in particular, are used as attachment devices for a wide variety of assembly and manufacturing applications, such as interior or exterior automotive mounting of panels and moldings. In a variety of such applications, adhesion to rough or irregular surfaces is desired or necessitated. Under these circumstances, thicker conformable pressure sensitive adhesive foams generally outperform thin pressure sensitive adhesives (such as those less than 125 microns thick). However, as applications for pressure-sensitive adhesives have increased substantially in recent years, performance requirements have become increasingly demanding. In particular, many applications require pressure sensitive adhesives to support a load at elevated temperatures, typically in the range of from 70° C. to 90° C., for which high cohesive strength systems are required.
While the use of crosslinking post-processing step, in particular chemical crosslinking, is generally recognized to provide improved mechanical properties to adhesive compositions, it also recognized that in some circumstances, the use of a crosslinking step may also detrimentally affect tackiness and adhesions characteristics of the resulting adhesive compositions and significantly compromise the ability of an adhesive composition to develop or maintain acceptable pressure sensitive adhesive properties. WO 02/34859-A1 for example describes a process for preparing hot melt adhesive compositions which obviates the need for a crosslinking post-processing step.
Moreover, the pressure sensitive adhesive materials known in the art do not often provide sufficient tack to various types of substrates, in particular the so-called LSE and MSE substrates, i.e. substrates having respectively a low surface energy and a medium surface energy. In particular, the peel force or shear resistance on these challenging-to-bond substrates do not often fulfill the requirements, especially under environmental stress like altering temperatures and humidity.
In addition to increasing performance requirements with regard to pressure sensitive adhesives, volatile organic compounds (VOC) reduction regulations are becoming increasingly important in particular for various kind of interior applications (occupational hygiene and occupational safety) such as e.g. in the construction market or in the automotive or electronics industries. Known acrylate-based pressure sensitive adhesives typically contain notable amounts of, low molecular weight organic residuals, such as un-reacted monomers arising from their polymerization process, polymerization initiator residuals, contaminations from raw materials or degradation products formed during the manufacturing process. These low molecular weight residuals qualifying as VOC may diffuse out of the adhesive tape and can be potentially harmful.
Without contesting the technical advantages associated with the pressure sensitive adhesives known in the art, there is still a need for a stable and cost-effective pressure sensitive adhesive foam providing excellent and versatile adhesion characteristics, in particular with respect to peel forces and static shear resistance both at room temperature and high temperature (e.g. 70° C.). Other advantages of the pressure sensitive adhesive (PSA) foams, assemblies and methods of the disclosure will be apparent from the following description.